yeah I don't have a solution to that problem right now, as mapped attributes
are only a class-bound concept and there is no concept of an arbitrary
attribute on an object that's not associated with a class-level mapped
attribute.
I think this problem long term would be solved more through some kind of
@property selector that works from a class and is not specific to mapping.
On Wed, Sep 23, 2020, at 8:51 PM, agrot...@gmail.com wrote:
> Understood. I was thinking though some sort of alias would be an interesting
> solution to the problem outlined about `contains_eager` as well:
> >Keep in mind that when we load only a subset of objects into a collection,
> >that collection no longer represents what’s actually in the database.
> In that rather than re-using a property which does have specific meaning, a
> new property could be created that explicitly has different meaning.
> On Wednesday, September 23, 2020 at 3:21:23 PM UTC-4 Mike Bayer wrote:
>> __
>> A.bs only goes to the "bs" collection on an A. there's no eagerloading that
>> puts the collection on some other arbitrary place.
>>
>> On Wed, Sep 23, 2020, at 6:03 PM, agrot...@gmail.com wrote:
>>> Cool, yes I think that is what I am looking for. Is there any way to alias
>>> the relationship (as read only) to: 1. allow for multiple different filters
>>> of the same property and 2. make sure when I read the value (in another
>>> place in the code), I have confidence the filter was applied?
>>>
>>> something like (I made up the syntax):
>>> q = session.query(A).options(selectinload(A.bs).and_(B.some_field ==
>>> value).as(f'bs_filtered_by_{value}')
>>> ...
>>> for a in q:
>>> for b in a.bs_filtered_by_xxxx:
>>> ....
>>>
>>>
>>> On Wednesday, September 23, 2020 at 12:21:41 PM UTC-4 Mike Bayer wrote:
>>>>
>>>>
>>>> On Wed, Sep 23, 2020, at 4:17 PM, agrot...@gmail.com wrote:
>>>>> I actually don't really care that much to have the attribute remain
>>>>> dynamic. In fact there is only one *specific* filtering that I want to
>>>>> apply to it, but that filtering will vary from (web) request to (web)
>>>>> request. This is what made me think of using contains_eager.
>>>>>
>>>>> Right now this is the best solution I have come up with, which is to
>>>>> define a temporary class that extends A and add to that class a new
>>>>> relationship with the custom filter applied. I then specify to
>>>>> selectinload that property. Is there a better way to do this?
>>>>
>>>> I would still use a separate relationship on the same class, you can
>>>> always make a @hybrid_property that switches between the two relationships
>>>> depending on what you want to do.
>>>>
>>>> In version 1.4, which will be in betas as soon as I can get a huge amount
>>>> of new docs written, you will have a potentially better option for this
>>>> which is the PropComparator.and_() operator. you can play with this now
>>>> from git master if you were interested:
>>>>
>>>> https://docs.sqlalchemy.org/en/14/orm/loading_relationships.html#adding-criteria-to-loader-options
>>>>
>>>> that way you'd say:
>>>>
>>>> session.query(A).options(selectinload(A.bs).and_(B.some_field == value))
>>>>
>>>> that might be what you're waiting for here
>>>>
>>>>
>>>>>
>>>>>
>>>>> # Run this code for each web request, reading some_field_value from the
>>>>> value the client specifies in the request:
>>>>> some_field_value = ...
>>>>>
>>>>> class ATmp(A):
>>>>> bs_temp = relationship(
>>>>> lambda: models.B,
>>>>> primaryjoin=(
>>>>> (models.A.id <http://models.a.id/> == models.B.a_id)
>>>>> & (models.B.some_field == some_field_value)
>>>>> ),
>>>>> )
>>>>>
>>>>> q = return db.session.query(ATmp).options(selectinload(cls.bs_temp))
>>>>> # iterate over the q (which in iterable of "A"s) and for each A, iterate
>>>>> over the bs_temp, which is a filtered collection of Bs.
>>>>>
>>>>> This also leads to a warning:
>>>>> SAWarning: This declarative base already contains a class with the same
>>>>> class name and module name as my_app.graphql.queries.ATmp, and will be
>>>>> replaced in the string-lookup table
>>>>>
>>>>> though it does seem to work (I am able to avoid n+1 and do the filtering
>>>>> in the DB).
>>>>> On Wednesday, September 23, 2020 at 8:10:44 AM UTC-4 Mike Bayer wrote:
>>>>>>
>>>>>>
>>>>>> On Wed, Sep 23, 2020, at 5:43 AM, agrot...@gmail.com wrote:
>>>>>>> Let's say I have a model with a one to many relationship as such:
>>>>>>> class A(Base):
>>>>>>> id = ...
>>>>>>>
>>>>>>> class B(Base):
>>>>>>> id = ...
>>>>>>> some_field = ....
>>>>>>> a_id = Column(ForeignKey(A.id)...
>>>>>>> a = relationship(A, backref=backref('bs', lazy='dynamic'))
>>>>>>>
>>>>>>> I can define a method on A:
>>>>>>> class A(Base):
>>>>>>> ...
>>>>>>> def get_b_with_some_field(self, some_field):
>>>>>>> return self.bs.filter(B.some_field==some_field)
>>>>>>>
>>>>>>> to get all b's that have a certain value of `some_field`.
>>>>>>>
>>>>>>> Is there any way to accomplish this with eager fetching to avoid the
>>>>>>> n+1 select problem that will occur if I want to query for a lot of `A`s
>>>>>>> and then iterate over the collection and for each call
>>>>>>> `get_b_with_some_field` with the same value of `some_field`?'
>>>>>>
>>>>>> if you want to have that attribute remain on "dynamic" then you'd need
>>>>>> to define a second relationship where you can use normal eager fetching
>>>>>> strategies, and then use that for those cases.
>>>>>>
>>>>>> IMO "dynamic" is not really worth it, you can get the same queries more
>>>>>> programmatically by using query(B).filter(with_parent(some_a, A.bs)).
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> One option is to to change relationship to from `lazy='dynamic'` to
>>>>>>> `lazy='subquery'` or ``lazy='selectin'`` and then implement the
>>>>>>> filtering in `get_b_with_some_field` in Python. This will address the
>>>>>>> n+1 select problem, but will cause pulling extra data from the database
>>>>>>> (and extra work in Python).
>>>>>>>
>>>>>>> I thought `contains_eager`
>>>>>>> <https://docs.sqlalchemy.org/en/13/orm/loading_relationships.html?highlight=contains_eager#using-contains-eager-to-load-a-custom-filtered-collection-result>
>>>>>>> might be relevant; however, I only see it being mentioned in the case
>>>>>>> of joined loads.
>>>>>>>
>>>>>>> The reason I am looking for this functionality is I am defining a
>>>>>>> graphql API that looks like the following:
>>>>>>> type query {
>>>>>>> as: [A!!
>>>>>>> }
>>>>>>>
>>>>>>> type A {
>>>>>>> ...
>>>>>>> bs(some_field: String): [B!]!
>>>>>>> }
>>>>>>>
>>>>>>> type B {
>>>>>>> ...
>>>>>>> some_field: String!
>>>>>>> }
>>>>>>> where I would like to be able to specify a filter on the `bs`
>>>>>>> relationship from `A`. I would ideally like to 1. avoid the n+1 select
>>>>>>> issue and 2. perform the some_field filtering at the database level,
>>>>>>> and 3. leverage as much of the ORM as possible ;-)
>>>>>>>
>>>>>>> Is it possible to do this within SQLA?
>>>>>>>
>>>>>>> --
>>>>>>> SQLAlchemy -
>>>>>>> The Python SQL Toolkit and Object Relational Mapper
>>>>>>>
>>>>>>> http://www.sqlalchemy.org/
>>>>>>>
>>>>>>> To post example code, please provide an MCVE: Minimal, Complete, and
>>>>>>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
>>>>>>> description.
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>>>>>>>
>>>>>>> <https://groups.google.com/d/msgid/sqlalchemy/55c7b3a6-bfee-45b6-83bc-25185bf7af87n%40googlegroups.com?utm_medium=email&utm_source=footer>.
>>>>>>
>>>>>
>>>>> --
>>>>> SQLAlchemy -
>>>>> The Python SQL Toolkit and Object Relational Mapper
>>>>>
>>>>> http://www.sqlalchemy.org/
>>>>>
>>>>> To post example code, please provide an MCVE: Minimal, Complete, and
>>>>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
>>>>> description.
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>>>>>
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>>>>
>>>
>>> --
>>> SQLAlchemy -
>>> The Python SQL Toolkit and Object Relational Mapper
>>>
>>> http://www.sqlalchemy.org/
>>>
>>> To post example code, please provide an MCVE: Minimal, Complete, and
>>> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
>>> description.
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>>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
>
> To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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--
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http://www.sqlalchemy.org/
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